A drilling tool of this kind may be configured as a core drill or core bit, but also as a rasp, and often requires a center drill for precisely placing the desired drilling or milling holes. Moreover, the center drill then extends coaxially to the large drill and protrudes with respect to the hard metal plates or tungsten carbide tips of the core drill.
In this approach that for example has prevailed for the placement of circular cylindrical recesses for the accommodation of switches and plugs in stone, concrete and brick or for wall breakthroughs, the center drill is involved in the drilling progress and comprises a hard metal plate or tungsten carbide tip in a manner known per se. It has been known for a long time to use a standard SDS-plus shaft for imparting a rotary driving motion to the center drill and for the accommodation thereof in the large drill, said standard SDS-plus shaft combining rotary driving grooves and guide grooves that act in the axial direction.
An approach of this kind is evident for example from DE-U-85 21 577. In this approach, two opposed threaded pins extend into the rotary driving grooves of the center drill; additionally, a locking element is provided as pull-out protection.
This approach has not prevailed in construction site practice. The worker in charge of the placement of the desired drill holes, often simply lets snap in the center drill into the locking element and does not care about the position of the one or more threaded pins. The center drill in this position has already snapped in and in this respect does not slip out when the worker holds the hammer drill with the center drill downward. Since the locking element is not intended for imparting the rotary driving motion, the center drill rotates in this position during the drilling operation in its drill mount or receptacle in the large drill, thereby resulting in the fast wear of the locking element and the center drill at its shaft.
Moreover, a certain experience is required for the setting of the threaded pins and a strict adherence to the standard operation procedures that are often not adhered to by the workers at construction sites. Namely, the threaded pins must be screwed into the rotary driving grooves as far or deep as possible, but must not block the center drill in a gripper-like manner. Through this, the threaded pins would be overstrained and would either break or at least be subjected to wear. It has also been observed that the quite thin threaded pins are quickly sheared off by the considerable torque exerted thereon at the front end thereof, so that the center drill in turn is free to rotate within its drill mount.
The disadvantages of the established solution have led to the taking into account of alternative solutions or approaches. About 10 years later it has been suggested to provide the center drill—instead of using the two pins that protrude into opposite grooves of the center drill—with a bevel which is to serve as a rotary drive for which purpose a pin that is guided at a clip, extends transversely to the center drill in a respective guide rail at the shaft of the large drill. This solution indeed does not lead to a wear of the center drill. However, in practice, it has also proved to be disadvantageous, since with this embodiment the large drills itself surprisingly often broke, probably because of the weakening at the shaft root due to the aperture mount for mounting the pin.